A significant obstacle for the successful management of patients with colorectal cancer is intrinsic drug resistance or, in patients who respond to chemotherapy, acquired drug resistance. Drug resistance can occur through a variety of mechanisms, including alterations in drug influx, drug efflux, intracellular metabolic activation, and intracellular catabolism, or through alterations in the drug's target. In addition, alterations in genes involved in the regulation of the cell cycle or in DNA damage repair may result in a cell becoming resistant to chemotherapy. In this chapter, the mechanisms of action and the mechanisms of resistance to the fluoropyrimidines and raltitrexed (Tomudex; Zeneca Pharmaceuticals, Wilmington, DE) are reviewed, focusing on newer studies using gastric and colorectal tumor samples obtained from patients. Clinical trials using this new information are anticipated.